[Lf] Higher L - higher ERP ???

[Andre' Kesteloot]

Vernall wrote:

> Mike and others,
>
> OK on the elevated loading coil.  I'm not convinced that hard results
> support your claims:
>
> > Yesterday, I reported that I had increased my mast-head
> > inductance so that the antennna was almost self-resonant. The new
> > coil introduced resistive losses and the current reduced, but reports
> > were better than before.
>
> If I recall, you said that the changed current distribution was an
> explanation for the reduced value of current at the feedpoint, which was
> incorrect.  Current is dependent on "loop resistance" at resonance.
>
> > Last night, Jim, M0BMU measured my field strength compared to a
> > measurement taken two weeks ago. It was 0.9dB up.
>
> That is very similar to some tests I did some time back, at 181 kHz, with a
> temporary top loading.
>
> > This does not account for the greatly improved reports,
>
> Quite so.  0.9 dB hardly shifts the S meter.
>
> >  but it does
> > show that with less current and more resistive losses, the only way
> > the ERP could have increased is if the 'effective height' had
> > increased.
>
> Yes.  The net result of decreased current because of increased loop
> resistance (a top loading coil necessarily needs higher inductance (more
> wire) than a base loading coil) and the improved current distribution in the
> vertical wire.  The effective height factor is just winning out over current
> reduction, by some 0.9 dB.
>
> >Field strength is of course directly proportional to current
> > squared multiplied by effective height (h) squared.
>
> True for radiated power.  Field strength is directly proportional to the
> product of current and effective height (amp metres).
>
> > At last here is real evidence that the elevated coil really does
> > increase the 'h' part of the equation. Several of us were sure that it
> > did, and several have noticed improvements in our signals when
> > using elevated coils, but the evidence has always been anecdotal.
>
> And still appears to be the case.  The same "gain" should be observed on
> transmit and receive (being wary that local noise could be different at each
> end of a path).
>
> There is still the matter of adequately housing the elevated loading coil
> and sustaining good insulation at high RF voltages.  The weight of a coil
> usually means it needs to be supported by a tower or mast.  Antenna
> modelling shows that if the vertical feed to a T antenna is near metalwork,
> it caused shunt capacitance that pulls down the gain by about 1 dB.  Tuning
> (and re-tuning) of the loading coil is somewhat inconvenient when it up in
> the air.
>
> One situation not amenable to modelling (with NEC-2) is when there is
> clutter around the vertical wire.  It seems that clutter causes
> disproportionate losses from "soakage" as the high field strength part of
> the wire rises above ground.  Some experimenters have reported significant
> improvement by relocating the "upwire" to being in a clearer environment.  A
> similar effect would arise with an elevated loading coil, as the potential
> on the "upwire" is considerably lower than for a base loading coil
> installation.  However, all of these environmeX-Mozilla-Status: 0009ow up in
> impedance data and gain measurements.
>
> I have observed the construction technique a number of LF NDB stations, from
> visits and photographs.  None have elevated loading coils.  All have clear
> sites.
>
> I'm not against experimentation and developing better antennas that fit in
> our back yards.
>
> 73, Bob ZL2CA